I made it to Charlottesville, and am all checked in to the Econo Lodge, which is a little more Econo than I was expecting. It’s an old-school motel, with rooms that open right into the parking lot, the sort of place where the towels are tiny and scratchy, the pillows and mattresses are thin, and the tap water tastes like cigarette smoke. Still, it’s not like I’m going to be doing much more than sleeping here.

I promised some entertainment in my absence, so this seems like a good time for a Dorky Poll. I’m going to be spending a lot of time over the next few days listening to talks about the interaction between light and matter, so here’s an esoteric question:

Which do you prefer: transverse waves, or longitudinal waves?

As always, leave your answers in the comments. If you’re confused about the question, I’ll put some explanation below the fold.

Waves are a disturbance in some medium (using a broad definition in which “the electromagnetic field” counts as a medium), and there are two main types of disturbance: the disturbance can be in the direction of motion of the wave (“longitudinal”), or perpendicular to it (“transverse”).

Sound waves are logitudinal. When you hit a wrong key, the speaker in your computer vibrates back and forth, pushing the air molecules next to it back and forth. These bump into air molecules a little further out, which bump into molecules further out, and so on until the wave reaches your ear. None of the molecules move all that far, but they shift back and forth a little bit along the line between the speaker and your ear.

Light waves are transverse waves. If a light wave is moving from left to right, it creates an electric field that oscillates up and down, and a magnetic field that oscillates in and out of the page (assuming, of course, that we’re in a classical limit where it makes sense to talk about light as a wave…). Particles set into motion by the wave will move in a direction perpendiculr to the line between the light source and your eye.

Comments

Personally, I like electromagnetic waveguide modes, which have both a transverse and longitudinal component. But, then again, some of my colleagues are regularly vexed by my refusal to take a stand on some issues…

In my line of work, longitudinal waves are much easier to deal with. Transverse waves get you into messy sums over infinite cyclotron harmonics of terms that involve lots of special functions, some of them not covered in Abramowitz and Stegun. Not that you avoid the special functions with longitudinal waves, but at least you don’t have to compute an infinite series of them.

It took me quite a while until I had an idea of how longitudinal waves work at all, and still occasionally find them difficult to imagine. Transverse waves are much more appealing. And, as Lethe pointed out, they are just cooler. Ever seen a longitudinal wave propagate through intergalactic vacuum?

As a geologist, I have to go with transverse. While longitudinal (P-waves) are a necessity for determining earthquake epicenters, transverse waves (S, Rayleigh and Love) are where the magic happens. Rayleigh and Love waves are the slow, high amplitude waves that are responsible for the real damage.

As a photons-from-space kind of guy, I’m gonna have to with transverse as well.
Also, (@Odysseus) I always get confused when I hear geologists talk about S- and P-waves. I can’t imagine a connection between the transverseness/longitudinality of a wave in an earthquake and its angular momentum. Though maybe I’m insufficiently imaginative?

When I taught lower-division physics, I always enjoyed doing the wave demos using a long slinky — the transverse waves were what you kind of expected to see, but for some reason I always found the longitudinal waves more unexpected and cooler. So consider this a vote for longitudinal.

As an aside, one of the things that I noticed when doing these demos is that the velocities of the transverse and longitudinal waves in a slinky are approximately equal. I recall doing a calculation that showed that this was true for a massive stretched spring with equilibrium length zero, which a slinky is to a reasonable approximation… I suspect that in part it is this equality of velocities that allows a slinky to do interesting things like going down stairs.

Ahem… Transverse all the way, baby! Not only are they much easier to draw and identify amplitude, but they can also be used to model longitudinal waves in diagrams such as like for standing waves in air columns.

Why yes, my two classes did just write their simple harmonic motion and waves final this morning, why do you ask? (And from the looks of things so far, aren’t doing all that well. Sigh)

My gut-level introduction to waves was also via a long Slinky. Very long – about 600 turns, would stretch the length of the 100′ corridor in the Physics building. Both forms are demonstrable. You do _not_ want to let go of your end.

But then came optics (transverse), and then came music (longitudinal in air, transverse on vinyl). Tough to choose, to each its niche. But longitudinal feel better, transverse look better.

2 And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.

3 And God said, Let there be light: and there was light.
King James translation, Genesis I:1-3.

I’m never happy with either/or. Can I go for the early stuff, fermions? That would be at least a little more explanatory than the Spirit of God. The transverse stuff is so new-fangled. I guess this comes out as spin-waves in Physics-speak.

The easiest thing to remember is that S is for sideways (or shear), since they represent waves with sideways (transverse) movement. That is how I remember it when mentioning it in class. You can also associate P with momentum if you imagine the momentum of the fracture producing compression waves, but that is not as good a cognate as S for sideways waves.

I like your idea though. I’ll have to ask a geologist if there are also d and f seismic waves, and see if they get it.

The real inquiry should be whether the stadium wave should be tolerated :^)

Circular polarization is wild stuff. I worked on microwave radios in the USAF and had a wonderful time working on some the steampunk radio sets. (Think SHF vintage 1948, no transistors!) When I got out and worked with some SRI guys, I was exposed to satellite communications for the first time.

Chad, circular polarization is not automatically a vote for transverse though, it can also be a vote for interesting things with mixed modes. Radio waves in magnetized space plasma. Crystal birefringence. Seismic shear wave splitting due to anisotropic stress fields. And so on.

Longitudinal and transverse in 3D just tend to be the most useful decompositions of oscillatory motion in many cases because of the beauty and (relative) simplicity of the theoretical description in a lot of situations.

The more subtle question: I can have waves where the oscillation is along the x-axis, waves where the oscillation is along the y-axis, waves where the oscillation is along the z-axis, and various combinations. How can I have (or why can I not have) waves where the oscillation is along the t-axis? You need not limit yourself to electromagnetism, nor earthquakes, nor sound. Show your work.

My fave waves are both longitudinal and transverse, but they oscillate transversely in (some would say more than a dozen) imaginary directions. The longitudinal aspect comes because of compression/stretching of the spacetime medium they move through, so their wavelength changes along their path.

In short; a complex probability wave through spacetime with gravity waves.

Hi Nerds(benevolent)-
What would cause different noises or sounds to get trapped on metal, wood, and other materials. I can release sounds from my home and car by shining a light or laser in fixed positions…
Yes….I need help, big time!1.5 years ago I was making $500k a year selling packaging as a economics grad. Something changed, as if Im being guided…or used… My brain cells need to come home. now!
Perhaps yall could help. Anyone know Steve Jobbs? I eyeyaeyae my eye phone

Books

You've read the blog, now try the books:

Eureka: Discovering Your Inner Scientist will be published in December 2014 by Basic Books. "This fun, diverse, and accessible look at how science works will convert even the biggest science phobe." --Publishers Weekly (starred review) "In writing that is welcoming but not overly bouncy, persuasive in a careful way but also enticing, Orzel reveals the “process of looking at the world, figuring out how things work, testing that knowledge, and sharing it with others.”...With an easy hand, Orzel ties together card games with communicating in the laboratory; playing sports and learning how to test and refine; the details of some hard science—Rutherford’s gold foil, Cavendish’s lamps and magnets—and entertaining stories that disclose the process that leads from observation to colorful narrative." --Kirkus ReviewsGoogle+

How to Teach Relativity to Your Dog is published by Basic Books. "“Unlike quantum physics, which remains bizarre even to experts, much of relativity makes sense. Thus, Einstein’s special relativity merely states that the laws of physics and the speed of light are identical for all observers in smooth motion. This sounds trivial but leads to weird if delightfully comprehensible phenomena, provided someone like Orzel delivers a clear explanation of why.” --Kirkus Reviews "Bravo to both man and dog." The New York Times.

How to Teach Physics to Your Dog is published by Scribner. "It's hard to imagine a better way for the mathematically and scientifically challenged, in particular, to grasp basic quantum physics." -- Booklist "Chad Orzel's How to Teach Physics to Your Dog is an absolutely delightful book on many axes: first, its subject matter, quantum physics, is arguably the most mind-bending scientific subject we have; second, the device of the book -- a quantum physicist, Orzel, explains quantum physics to Emmy, his cheeky German shepherd -- is a hoot, and has the singular advantage of making the mind-bending a little less traumatic when the going gets tough (quantum physics has a certain irreducible complexity that precludes an easy understanding of its implications); finally, third, it is extremely well-written, combining a scientist's rigor and accuracy with a natural raconteur's storytelling skill." -- BoingBoing